1. Technical Field
The present disclosure relates to cell configurations for an exhaust treatment device for use with internal combustion engines.
2. Background Art
Achieving low emissions, particularly low NOx (NO, nitric oxide, plus NO2, nitrogen dioxide) emissions from lean-burning engines, such as diesel engines is a challenge. SCRs (selective catalytic reductions) and LNTs (lean NOx traps) have been developed to treat NOx emitted from diesel engines. LNTs operate in a lean/rich cycle in which NOx is purged during lean operation and NOx is released and reacted in a shorter period of rich operation. A disadvantage of LNTs is that they significantly degrade diesel fuel economy. Although SCRs do not consume a large amount of extra fuel to react NOx, urea is supplied to the SCR to facilitate the reaction to treat NOx. For vehicular use, an onboard urea tank and delivery system is used.
While SCRs are very effective at converting NOx to N2 and O2 under steady state conditions, during transient conditions such as tip-ins (driver demand for a rapid increase in torque), a high concentration of NOx may pass through the SCR and may not be entirely reacted. SCRs have not achieved the extremely low NOx emission levels that have been achieved with gasoline engines using three-way catalysts, largely due to NOx slippage or breakthrough during transients. These NOx tip-in spikes are difficult to reduce due to slow kinetics and poor diffusion in the SCR device.